The present invention relates to laser processing of memory or other IC links and, in particular, to a laser system and method employing a burst of laser pulses having ultrashort pulse widths to sever an IC link.
Yields in IC device fabrication processes often incur defects resulting from alignment variations of subsurface layers or patterns or particulate contaminants. FIGS. 1, 2A, and 2B show repetitive electronic circuits 10 of an IC device or workpiece 12 that are commonly fabricated in rows or columns to include multiple iterations of redundant circuit elements 14, such as spare rows 16 and columns 18 of memory cells 20. With reference to FIGS. 1, 2A, and 2B, circuits 10 are also designed to include particular laser severable circuit links 22 between electrical contacts 24 that can be removed to disconnect a defective memory cell 20, for example, and substitute a replacement redundant cell 26 in a memory device such as a DRAM, an SRAM, or an embedded memory. Similar techniques are also used to sever links to program a logic product, gate arrays, or ASICs.
Links 22 are about 0.5-2 microns (xcexcm) thick and are designed with conventional link widths 28 of about 0.8-2.5 xcexcm, link lengths 30, and element-to-element pitches (center-to-center spacings) 32 of about 2-8 xcexcm from adjacent circuit structures or elements 34, such as link structures 36. Although the most prevalent link materials have been polysilicon and like compositions, memory manufacturers have more recently adopted a variety of more conductive metallic link materials that may include, but are not limited to, aluminum, copper, gold nickel, titanium, tungsten, platinum, as well as other metals, metal alloys, metal nitrides such as titanium or tantalum nitride, metal silicides such as tungsten silicide, or other metal-like materials.
Circuits 10, circuit elements 14, or cells 20 are tested for defects, the locations of which may be mapped into a database or program. Traditional 1.047 xcexcm or 1.064 xcexcm infrared (IR) laser wavelengths have been employed for more than 20 years to explosively remove circuit links 22. Conventional memory link processing systems focus a single pulse of laser output having a pulse width of about 4 to 20 nanoseconds (ns) at each link 22. FIGS. 2A and 2B show a laser spot 38 of spot size diameter 40 impinging a link structure 36 composed of a polysilicon or metal link 22 positioned above a silicon substrate 42 and between component layers of a passivation layer stack including an overlying passivation layer 44 (shown in FIG. 2A but not in FIG. 2B), which is typically 2000-10,000 angstrom (xc3x85) thick, and an underlying passivation layer 46. Silicon substrate 42 absorbs a relatively small proportional quantity of IR radiation, and conventional passivation layers 44 and 46 such as silicon dioxide or silicon nitride are relatively transparent to IR radiation. FIG. 2C is a fragmentary cross-sectional side view of the link structure of FIG. 2B after the link 22 is removed by the prior art laser pulse.
To avoid damage to the substrate 42 while maintaining sufficient energy to process a metal or nonmetal link 22, Sun et al. in U.S. Pat. No. 5,265,114 and U.S. Pat. No. 5,473,624 proposed using a single 9 to 25 ns pulse at a longer laser wavelength, such as 1.3 xcexcm, to process memory links 22 on silicon wafers. At the 1.3 xcexcm laser wavelength, the absorption contrast between the link material and silicon substrate 42 is much larger than that at the traditional 1 xcexcm laser wavelengths. The much wider laser processing window and better processing quality afforded by this technique has been used in the industry for about three years with great success.
The 1.0 xcexcm and 1.3 xcexcm laser wavelengths have disadvantages however. The coupling efficiency of such IR laser beams into a highly electrically conductive metallic link 22 is relatively poor; and the practical achievable spot size 38 of an IR laser beam for link severing is relatively large and limits the critical dimensions of link width 28, link length 30 between contacts 24, and link pitch 32. This conventional laser link processing relies on heating, melting, and evaporating link 22, and creating a mechanical stress build-up to explosively open overlying passivation layer 44. Such a conventional link processing laser pulse creates a large heat affected zone (HAZ) that deteriorates the quality of the device that includes the severed link.
The thermal-stress explosion behavior is also somewhat dependent on the width of link 22. As the link width becomes narrower than about 1 xcexcm, the explosion pattern of passivation layers 44 becomes irregular and results in an inconsistent link processing quality that is unacceptable and limits circuit density. Thus, the thermal-stress behavior limits the critical dimensions of links 22 and prevents greater circuit density.
U.S. Pat. No. 6,057,180 of Sun et al. and U.S. Pat. No. 6,025,256 of Swenson et al. more recently describe methods of using ultraviolet (UV) laser output to sever or expose links that xe2x80x9copenxe2x80x9d the overlying passivation by different material removal mechanisms and have the benefit of a smaller beam spot size. However, removal of the link itself by such a UV laser pulse requires the passivation material to be UV absorbing and is still a xe2x80x9cthermalxe2x80x9d process.
U.S. Pat. No. 5,656,186 of Mourou et al. discloses a general method of laser induced breakdown and ablation by high repetition rate ultrafast laser pulses.
U.S. Pat. No. 5,208,437 of Miyauchi et al. discloses a method of using a single pulse of a subnanosecond pulse width to process a link.
U.S. Pat. No. 5,742,634 of Rieger et al. discloses a simultaneously Q-switched and mode-locked neodymium (Nd) laser device with diode pumping. The laser emits a series of pulses each having a duration time of 60 to 300 picoseconds (ps), under an envelope of a time duration of 100 ns. Pulses having a duration time of 60 to 300 ps exhibit a xe2x80x9cthermalxe2x80x9d mechanism of material processing.
An object of the present invention is to provide a method or apparatus for improving the quality of laser processing of IC links.
Another object of the invention is to process links with bursts of ultrashort laser pulses that have a nonthermal interaction with the overlying passivation layer and the link material.
A further object of the invention is to employ the bursts of ultrashort laser pulses to process the links on-the-fly.
The present invention employs a burst ultrashort laser pulses to sever an IC link, instead of using a single multiple-nanosecond laser pulse of conventional link processing systems. The duration of the burst is preferably in the range of 10 to 500 ns; and the pulse width of each laser pulse within the burst is generally shorter than 25 ps, preferably shorter than or equal to 10 ps, and most preferably about 10 ps to 100 femtoseconds (fs). Because each laser pulse within the burst is ultrashort, its interaction with the target materials (passivation layers and metallic link) is not thermal. Each laser pulse breaks off a thin sublayer of about 100-2,000 xc3x85 of material, depending on the laser energy, laser wavelength, and type of material, until the link is severed. The number of ultrashort laser pulses in the burst is controlled such that the last pulse cleans off the bottom of the link leaving the underlying passivation layer and the substrate intact. Because the whole duration of the burst is in the range of 10 ns to 500 ns, the burst is considered to be a single xe2x80x9cpulsexe2x80x9d by a traditional link-severing laser positioning system. Thus, the laser system can still process links on-the-fly, i.e. the positioning system does not have to stop moving when the laser system fires a burst of laser pulses at each link.
In addition to the xe2x80x9cnonthermalxe2x80x9d and well-controllable nature of ultrashort-pulse laser processing, the most common ultrashort-pulse laser source emits at a wavelength of about 800 nm and facilitates delivery of a small-sized laser spot. Preferably, a diode-pumped, or diode-pumped solid-state continuous wave (CW) green pumped, mode-locked, solid-state laser is employed to generate the ultrashort pulses at conventional wavelengths or their harmonics.